This invention relates to a process for polymerizing trioxane, and particularly relates to a process for polymerizing crude trioxane made from an aqueous solution of methanol-containing formaldehyde by a conventional method or copolymerizing the trioxane with another monomer, characterized
cooling the trioxane to crystallize it, PA1 allowing the trioxane in a crystalline state to stand in an open system in an atmosphere of air or an inert gas to remove polyoxymethylene dimethoxide from the trioxane, PA1 melting the trioxane, and PA1 polymerizing the trioxane or copolymerizing the trioxane and another monomer.
In general, trioxane has been prepared by heating an aqueous solution of formaldehyde in the presence of an acid catalyst. Trioxane has been industrially prepared according to the steps of heating a 30-70% aqueous solution of formaldehyde in the presence of an acid catalyst to obtain a distillate containing 20-55% by weight of trioxane, 17-35% by weight of formaldehyde and 20-50% by weight of water, and extracting the distillate with a solvent which is insoluble or only slightly soluble in water to obtain trioxane from the distillate.
Furthermore, examples of processes for purifying the crude trioxane include a process for distilling the crude trioxane in the presence of an anionic ion exchange resin or a basic amine; a process for purifying the crude trioxane using activated alumina or molecular sieves; a process for extracting the tioxane with a solvent and removing the solvent from the distillate by washing, followed by distilling trioxane in the presence of an amine; and a process for removing impurities from the crude trioxane through chemical reaction. However, though in these processes for purification of trioxane, formic acid, water and formaldehyde are removed from the crude trioxane, polyoxymethylene dimethoxide formed as a by-product during preparation of trioxane is not be removed.
Polyoxymethylene dimethoxide is represented by the following formula EQU CH.sub.3 O(CH.sub.2 O).sub.n CH.sub.3 . . . (1)
wherein n is integer of 2-7. Polyoxymethylene dimethoxide is always formed as a by-produce when an aqueous solution of methanol-containing formaldehyde is heated in the presence of an acid catalyst to form trioxane.
Particularly, much polyoxymethylene dimethoxide having the formula EQU CH.sub.3 O(CH.sub.2 O).sub.n CH.sub.3 . . . (1)
wherein n is 2 or 3 is formed in this case. The polyoxymethylene dimethoxide acts as a chain transfer agent in case of polymerizing trioxane or copolymerizing trioxane and another monomer, thereby lowering the molecular weight of the resulting polymer or copolymer. Therefore, before trioxane alone is polymerized or trioxane is copolymerized with another monomer, it is necessary to remove polyoxymethylene dimethoxide from the trioxane before polymerization. In general, it is preferable that the concentration of the polyoxymethylene dimethoxide be reduced to less than 500 ppm, more preferably 200 ppm. However, the boiling point of polyoxymethylene dimethoxide having the formula EQU CH.sub.3 O(CH.sub.2 O).sub.n CH.sub.3
wherein n is 2 or 3 is near to that trioxane. In addition, the two compounds are similar to each other in chemical properties. So, the two compounds can not be separated by distillation, adsorption, extraction or reaction with another compound. Therefore, when trioxane is polymerized or copolymerized by the prior art process, lowering of the molecular weight of the resulting polymer or copolymer cannot be avoided.
When an aqueous solution of formaldehyde having lower methanol content, such as paraformaldehyde or alpha-polyoxymethylene having formaldehyde content of more than 85% and methanol content of less than 1% is heated, trioxane having lower polyoxymethylene dimethoxide content can be obtained. But such paraformaldehyde and alpha-polyoxymethylene are costly. To use these raw materials industrially is not efficient. In general, an aqueous solution having 30-40% by weight of formaldehyde and 1-8% by weight of methanol has been used as a raw material for preparing trioxane. So, formation of polyoxymethylene dimethoxide as a by-product is unavoidable. In the above mentioned formula (1), the compound wherein n is 1 is methylal. Though methylal acts as a chain transfer agent in case of polymerizing trioxane, it can be easily removed through distillation.